Hydrostatic transmissions have many uses, including the propelling of vehicles, such as mowing machines. A typical hydrostatic transmission system includes a variable displacement hydraulic pump connected in a closed hydraulic circuit with a fixed or variable displacement hydraulic motor. The closed hydraulic circuit includes a first conduit connecting the main pump outlet with the motor inlet and a second conduit connecting the motor outlet with a pump inlet. Either of these conduits may be the high pressure line depending upon the direction of pump displacement from neutral.
For most applications, the pump is driven by a prime mover, such as an internal combustion engine or an electrical motor, at a certain speed in a certain direction. Changing the displacement of the main pump will change its output flow rate, which controls the speed of the motor. Pump outflow can be reversed, thus reversing the direction of the motor. In a vehicle, the motor is often connected through suitable gearing to the vehicle's wheels or tracks. In some vehicles, such as zero-turn-radius mowers, separate hydraulic pumps and motors are used to independently drive separate wheels of an axle.
To park the vehicle, a brake apparatus is typically provided in the transmission to restrict or prevent rotational motion of a rotatable member, such as the output shaft, connected to the wheels. In some brake assemblies, a hub or rotor is connected to the rotatable member and a friction member engages the hub to restrict motion through frictional brake torque. In some brake assemblies, a locking mechanism mechanically interferes with the hub to restrict or prevent movement and provide the required brake torque.
Known brake assemblies typically require a gear reduction to amplify the brake torque, or may require larger than desired actuation forces to engage the hub. This is particularly problematic when the vehicle is parked on an incline or grade. Furthermore, such brake assemblies may require an external force, for example a hydraulic caliper or spring, to maintain load-holding and may also require the user to exert relatively high forces to disengage the brake, especially when parked at an incline.